This study is a Phase II treatment for patients with low grade lymphomas or chronic lymphocytic leukemia and includes both treated and untreated patients. The immunotoxin is anti-B4-blocked ricin, and includes a conjugate between a monoclonal antibody that recognizes CD19 (which is present on most B-cell malignancies) and blocked ricin (in which the B- chain of the toxin has been blocked). The immunotoxin is given by 24-hour continuous infusion for seven consecutive days. Treatment is repeated every 14 days. Clinical response is the endpoint. To date, six low grade lymphoma patients have been entered onto this study. The first patient received anti-B4-blocked ricin 50 mcg/kg/day, but the infusion was stopped prematurely (on day 5) due to unacceptable toxicity. The patient died on day seven of the study with refractory hypotension likely due to capillary leak syndrome. Subsequently, the dose of the immunotoxin was decreased to 30 mcg/kg/day and adjusted per lean body weight rather than actual body weight. An additional 5 patients have been treated at this lower dose level with only moderate toxicity. Among the six patients treated, five had stable disease and one had progressive disease. Three patients continue to have stable disease and have received no further therapy. Two patients have died with progressive lymphoma.